Britain's Secret Intelligence Service (also commonly known as MI6) was born a century ago amid fears of the rising power of other countries, especially Germany. The next 40 years saw MI6 taking an increasingly important if largely hidden role in shaping the history of Europe and the world. MI6's early days were haphazard but it was quickly forged into an effective organization in the crucible of World War I. During these war years, MI6 also formed ties with the United States in a relationship that would become vital to both countries' security as the century progressed. These early years also saw the development of techniques that would become plot devices in a thousand books and films—forgery, invisible ink, disguises, concealing mechanisms, and much more. The interwar years were nominally peaceful, but Britain perceived numerous threats, all of which MI6 was expected to keep tabs on. The outbreak of World War II once again caught MI6 off balance, and high-profile blunders (and the memoirs of MI6 operatives like Graham Greene) created an impression of ineffectiveness. Even so, the service was pioneering cryptography at Bletchley Park and devising the very methods and equipment that would inspire Ian Fleming's novels. This fascinating and thorough account draws on a wealth of previously inaccessible archival materials to document the inner workings of the world's first spy agency before it reinvented itself for the Cold War era.